I've just found this on Twitter, posted on 1st May:
ThePavaoQuartet: Shame we had to cancel our Radio 2 performance last week - Natalia was stuck abroad due to the volcano!
The Pavão Quartet were due to record for Friday Night is Music Night on Wednesday 21st April. If Raven replaced them, the programme may be broadcast during the next few weeks. Could you or Grant ask Raven on Twitter, please?